A finback whale estimated to be about 30-40 feet long has washed up dead on a Long Island beach.

Kim Durham of the Riverhead Foundation for Marine Research and Preservation says aerial photos from Atlantic Beach in Nassau County show the whale appears to be the same one that experts have been tracking since Sunday.

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A finback whale estimated to be about 30-40 feet long has washed up dead on a Long Island beach. (Published Wednesday, June 8, 2011)

She says that whale was first seen near Long Branch, N.J., and experts had been watching for where it would surface. Atlantic Beach is on Long Island's south shore, about 30 miles from Manhattan.

The whale could have been struck by a boat or killed by an unknown disease, Durham added.

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Onlookers were taken aback by the sight of the dead whale.

"I'm shocked. I never saw anything like that before," said Catherine Ferrante, who was on the beach Wednesday.

It had been dead for several days. The whale's carcass was first seen floating off Long Branch, N.J. on Sunday, according to Durham.

"You go swimming in the water and never think there are these huge mammals swimming around," said Linda Bucher.

But whales have been seen closer to shore this year, said Durham, apparently because they are following their food supply of fish.

After the animal autopsy, the whale's carcass is expected to be removed from the beach and burned.